A point diffraction interferometer (PDI) utilizes a standard reference spherical wave generated by a diffraction of a pinhole to implement an interference measurement. The measuring method by joining sub-apertures pinhole divides the measured surface to be a plurality of sub-apertures pinhole to be tested and then stitched, which may improve the lateral resolution of detection.
The proposal of the point diffraction interferometer solves a problem of machining the reference surface in a measurement with a high precision. Its primary feature is not to utilize a conventional reference surface, from the point of view of wave optics, which eliminates the limitation of the machining level of the reference surface on the measuring precision by utilizing a diffraction of a pinhole to generate an ideal reference spherical wave and makes it possible for a measurement with a high precision in scale of sub-nanometer.
In 1933, W. Linnk firstly propose to utilize an ideal spherical wave generated by a diffraction of a pinhole as a reference wavefront of the interferometer to provide a preform of a point diffraction interferometer. However, due to technical limitation at that time, it is not actually applied to measurement. In 1975, R. N. Smartt and W. H. Steel formally set forth a principle and application of a point diffraction interferometer in their publications and establish a basis for the development of a modern point diffraction interferometer. Their proposed point diffraction interferometer has a main portion of a thin film with a transmissivity of about 1% on which there is a very small pinhole. The lower transmissivity is to make light intensities of the two light beams to be close to each other. When the focused measured light passes through a plate of the thin film, a surface shape of the transmitted light wave is kept to be not changed except for the decrease of energy. In dispersion spot region with a aberration, a diffraction occurs at the pinhole nearby the focus to generate an ideal standard spherical wave as a reference light wave in the measurement, which forms an interference fringe along with the transmitted measured light. Information about the measured wave front may be obtained by analyzing a shape of the interference fringe. Such an interferometer has a simple structure and a basic principle. Since a common light path arrangement is utilized, the influence from the environment is small. Its disadvantage is a lower usage of the light energy and a measurement of phase shift can't be done at the same which, so that it is difficult to improve the precision.
In 1996, H. Medecki, E. Tejnil, et. al. of Lawrence Berkeley National Laboratory of USA propose a concept of phase-shifting point-diffraction interferometer (PS-PDI). That is to say, on the basis of the point diffraction interferometer, a diffraction grating is introduced to function as a dispersing element and a translucent mask of an imaging plane is replaced with an opaque mask, so that the property of the point diffraction interferometer is greatly improved. For a basic structure of such a point diffraction interferometer, when an irradiation spherical wave is incidence on a phase shift grating to form different orders of diffraction; they passes through the measured system and are focused onto different positions of the image plane. A spatial filter is positioned on the image plane so that the light of the zeroth diffraction order carrying information about the measured system directly passes through the spatial filter via a square hole, the light of the first diffraction order is diffracted and filtered by the pinhole to generate an ideal reference spherical wave; and the light of the remaining other diffraction orders are absorbed. Thus, the interference fringe of the two light beams is obtained on the detector (CCD). When the grating is moved along an up-down direction, a phase change is occurred between the two light beams so that a phase shift and interference measurement can be implemented.
In order to meet the requirement of measurement for an extreme ultraviolet (EUV) photolithography system, since 1996, the researchers from the Lawrence Berkeley National Laboratory employ a synchrotron radiation light source of 13.4 nm to successfully develop an EUV phase-shift point-diffraction interferometer, which improves the measuring precision of the EUV system to an order of sub-nanometer and eliminates barrier for the development of the EUV photolithography.
Since the end of the last century, researchers in Japanese started to research a point diffraction interferometer. In order to detect an EUV photolithography system, association of super-advanced electronics technology (ASET), Nikon Co. and the like research the point diffraction interferometer. One type of the employed point diffraction interferometer utilizes a reflecting plate with an pinhole. One part of the diffraction spherical wave of the pinhole functions as a reference light wave, and the other part of the diffraction spherical wave is reflected by the measured plane and the reflecting plane, and then interference with the reference light. Since such an arrangement is not a system with a common light path, the requirements on the coherence of the light source and the stability of the environment are higher, and all of the measurements should be done at an anti-vibration and nitrogen-filled environment.
With a continued development of science and technique, an optical system with a large-diameter aperture are widely applied to a high technique filed such as astro optics, space optics, detection and identification of spatial object, inertial confinement fusion and so on. Thus, the manufacturing of the optical element with a large-diameter aperture needs detection methods and equipments which adapts such an optical elements.
At present, an optical element with a large-diameter aperture usually employs a phase shift interferometer, and its quality of the machined surface is determined by the phase shift interferometer. Thus, it is desired to have a standard surface shape, the size of which is identical to or larger than that of the measured element. However, for such a standard surface with a high precision, it is difficult to manufacture for a long period of manufacturing and at a high cost, which virtually increases cost and difficulty of detection. In order to find a detection means at a low cost, a technical solution of stitching sub-aperture is developed in 1980s overseas. That is to say, an interferometer with a small-diameter aperture, a high precision and a high resolution is used to recovery a wave front phase data for an optical element with a large-diameter aperture by a corresponding stitching technique. Such a technique is a novel detection means with a high precision and a large-diameter aperture, which reserves a high precision of interference measurement and avoids use of a standard wave surface, the size of which is identical that of a full aperture of an pinhole so as to greatly reduce the cost and to obtain a high frequency information cut off by the interferometer with a large-diameter aperture.
The concept of measurement of sub-aperture is proposed in 1982 by C. J. Kim in Arizona optical center, USA, which utilizes an array of reflecting mirrors with a small-diameter aperture to replace the reflecting mirror with a large-diameter aperture so as to implement a self-collimated inspection of a parabolic mirror. At early days of 1990s, such a technique is gradually applied to a stage of application and research with the continued developments of computer control and data processing technique. S. T. Theodore applies the measurement of sub-aperture to an improved Ritchey-common configuration which has a shorter light path than a conventional Ritchey-common configuration and may effectively reduce influence of atmospheric disturbance. The diameter of light beam returned to the optical element is smaller than that of the measured light beam.
The stitching algorithm developed during such a period minimize mismatch of the overlapped regions of the plurality of sub-apertures to obtain a reconstruction of the surface shape for a full pinhole with a high spatial resolution. In addition, an introduction of averaging of error greatly improves the precision of the stitching algorithm. These relevant techniques are mainly applied to an inspection of a surface shape with a large-diameter aperture so as to extend a lateral dynamic range.
In 1997, M. Bray manufactured a practical sub-aperture stitching interferometer for detection of an optical plane element with a large-diameter aperture. In several subsequent years, M. Bray introduced a concept of power spectral density to analyze the characteristic of a stitching interferometer. It indicates that it may accurately describe a stitching “noise” caused by an pinhole edge effect.
In 2003, the QED Technology Co. of USA successfully developed a SSI automatically stitching interferometer which is capable of detecting a plane surface, a spherical surface and an aspheric surface having an appropriate deviation and an aperture within 200 mm under a high precision. Such a stitching algorithm inherits an advantage of the forepart algorithm and further compensates a system error besides the relative adjustment error corrected by the conventional algorithm, which further improves the precision of stitching.
In China, a research on a measurement of sub-apertures starts at beginning of 1990s and is mainly used for detection of an optical plane element having a large-diameter aperture. Researchers in Nanjing University of Science and Technology apply the measuring technique of a sub-apertures to a phase shift plane interferometer, thereby to extend the measuring range of the pinhole from about 250 mm to about 500 mm.
During the last a few years in 1990s, researchers in State key laboratory on modern optical instruments in Zhejiang University utilized an sub-aperture detection method to verifies a RC optical system for some earth resources satellite and proposes a method of analyzing the stitched object by a function to reduce the error accumulation and error transfer caused by pair wise stitching between the sub-apertures.
As can be seen from the development and principle of the point diffraction interferometer, the point diffraction interferometer utilizes an ideal spherical wave generated by an pinhole to measure. Since the resolution of a detector is limited, the resolution of measurements is decreases as the detection aperture is increased. It is desired a higher lateral resolution for more and more complex surface shapes.